The present invention relates to an electronic sensor for detecting chemical compounds, and, in particular, a sensor capable of detecting organophosphorus compounds and chemical warfare nerve agents.
In the course of daily activity, the living and working environment exposes man to a variety of toxic gases and vapors. Today, the circumstances in many industrial environments is particularly precarious, and this situation has motivated the establishment of specific exposure level guidelines and monitoring schemes for ensuring personal safety. One particular group of contaminants are the organophosphorus pesticides and structurally affiliated chemical warfare nerve agents. Adding to their impact, these organophosphorus compounds are synthesized and valued for their deleterious effect and persistence.
Several types of air pollution monitoring systems are available which lend themselves to varying degrees of portability. At one end of the technology spectrum, a sample of the atmosphere is taken and sent to a laboratory for analysis. This process is inconvenient, after the fact, expensive, and time consuming. At the other end of the technology spectrum, direct monitoring instruments are used to monitor a specific contaminant in a large work area. However, these systems tend to be bulky, complicated, expensive to purchase, operate and maintain, and fail to provide information on individual exposure.
It is well documented that a material's surface and electronic properties are strongly influenced by ambient conditions, but little progress has been made exploiting this phenomenon for the practical application of toxic gas detection. The primary problem involves identifying the fundamental electronic mechanisms and processes by which certain materials respond to gas ambients. A secondary problem encompasses the utilization of materials compatible with conventional integrated circuit fabrication technology, but yet retain sensitivity and selectivity to the toxic gas of interest.
One type of personal detector for nerve agents is a litmus type paper that is issued to GIs when in areas of potential danger. Nerve agents that are applied in a gaseous form, invisible to the eye, obviously would not be detected until it was too late to take effective action. The only warning would be the presence of bodies without injuries in an area. Typically, nerve agents are applied in an aerosol form from a bursting ordinance round or sprayed from an aircraft. The droplets would be found on the surface of clothing, vehicles, aircraft, etc. The litmus type paper is rubbed against the surface and if the particular agent is present, the paper changes color. Based upon the type of nerve agent found, the proper antidote can be administered. Given the proper warning, chemical warfare clothing can be used for protection until the agent is removed or neutralized. The above warnings, such as dud sounding ordinance, fog in low lying areas, stricken personnel, aircraft spraying may come too late for the GI in the field away from area electronic agent detectors.
Another type of detector has a housing for holding and protecting an electronic circuit and a pair of deflectable micromechanical cantilevers driven by oscillators. One of the pair of cantilevers is coated with a chemically selective substance to form an end-mass load. As the nerve agent of concern is absorbed by the selective substance, the end-mass changes and so does the resonant frequency of that coated cantilever. The uncoated cantilever acts as a standard frequency source to which the coated cantilever is compared. These frequencies are input to a mixer circuit that outputs the difference frequency. The difference frequency, after going through pulse shaping, is input to counter circuits that determine if a set limit is exceeded and, if so, how much is the limit exceeded by as a function of time. This detector is further described in U.S. Pat. No. 4,549,427 which is incorporated by reference. These deficiencies have motivated a search for alternative devices.